This invention relates generally to d-c voltage converters, and more particularly to electronic watches or other miniature electronic devices provided with a d-c voltage source and a voltage converter adapted to increase or reduce the voltage thereof.
By the expression "miniature electronic device" as used herein is meant a battery-operated instrument or apparatus whose dimensions are so small that it can be carried about, the power consumption thereof being very slight. It is desirable that a device of this type include a compact d-c voltage converter unit having the smallest possible energy losses. A typical example of such a miniature electronic device is an electronic wristwatch. Other examples are digital-display thermometers and monitoring instruments which can be carried on the body such as pulse-rate meters or other small biomedical apparatus. Also included are miniature computers or the like. In general, the concern of the present invention is with miniaturized electronic devices in which, for some reason, the available d-c supply voltage must be increased or reduced with a minimum of losses.
Thus digital display instruments are known in which the voltage of the supply battery, usually amounting to 1.5 volts, is stepped up in order to actuate a display system having liquid-crystal or other electro-optical elements.
The problem of converting a d-c voltage in a manner maintaining high efficiency is particularly acute in those situations where only one low voltage power supply source is available in a miniature electronic device. Thus if the energy for operating a wristwatch is generated by thermal elements that exploit the temperature difference between the back of the watch case, which is warm when worn, and the colder outer part of the case which is insulated thermally from the back, the voltage produced thereby is minute. Even using a large number of such thermal elements connected in series, the resultant d-c voltage will still be at a very low level.
It is necessary, therefore, to step up the available d.c. supply voltage by converter means including a chopper, a transformer and a rectifier. The principle of converting a d-c voltage in this manner is well known, the alternating voltage induced in the secondary winding of the transformer being rectified by means of selenium cells, silicon diodes or the like. A converter arrangement of this type is disclosed in the Sutter application Ser. No. 968,694, filed Dec. 12, 1978 (now U.S. Pat. No. 4,214,434), whose entire disclosure is incorporated herein by reference.
It has been found, particularly in the case of thermoelectrically-operated wristwatches or other modern microelectronic devices, that rectification by means of diodes has fundamental disadvantages. Even in the case of integrated diodes, diodes characteristically have a threshold voltage of at least about 0.5 to 0.6 volts. This means that the diode conducts practically no current as long as the voltage applied to it is below this critical value. In the case of an electronic watch requiring a d-c voltage of 1.5 volts for the operation of the electronic circuits and of the stepping motor or digital display system, when this watch is powered by thermal elements or solar cells, one is compelled, when using diodes in the d-c converter to produce an alternating voltage having almost twice the voltage that would have been necessary had the diodes been responsive at much lower voltages.
The unfavorable ratio of diode threshold value to the required d.c. voltage (i.e., the d-c voltage that must be available at the output of the d-c voltage converter) in rectifier circuits for miniature electronic devices gives rise to a number of serious disadvantages. Since possibly more than one-half of the power given off by the voltage transformer is wasted as loss power in the diodes, a relatively large voltage transformer is required whose secondary winding must have a large number of turns. By reason of this requirement, the internal resistance of the secondary winding it also correspondingly increased.
Moreover, the need for a large step-up ratio reduces the efficiency of the transformer because of less favorable coupling factors. Aside from the concomitant increase in volume and the rise in the cost of the transformer, is the disturbing fact that in many cases the source of available d.c. supply voltage is excessively loaded so that the power yielded by the rectifier may be altogether insufficient to operate the device or the components of the device which are powered by the converted d-c voltage. Finally, with a large ratio of diode threshold voltage to rectifier output voltage, irregularities in the characteristics of the diodes as well as the dependence of their threshold value on the temperature adversely affect the stability or constancy of the rectified voltage.
The above-identified drawbacks militate against the production of an electronic miniature device whose operation is reliable and efficient.